The present invention relates to multilayer plastic containers for pressurized products which may be exposed to elevated temperatures and pressures, e.g., during pasteurization, and wherein the multiple layers are resistant to layer separation.
Juice drinks are normally filled bv one of three basic sterilization processes:
hot fill
pasteurization
aseptic fill.
Hot filling is not suitable for carbonated juice drinks due to the inability to maintain carbon dioxide (CO2) in solution at elevated temperatures. Aseptic filling of carbonated drinks is possible, but has certain disadvantages which include requiring high levels of capital investment, operating maintenance, and expertise. As a result, pasteurization is the preferred sterilization approach for carbonated juice drinks.
Prior art pasteurizable beverage containers are typically made of glass, because glass can withstand the extended high temperatures and high internal pressures of the pasteurization cycle. FIG. 1 illustrates graphically, as a function of time, the increasing internal temperature and pressure during a pasteurization cycle of a 16-ounce glass container, which has been filled with a juice product carbonated at 2.5 volumes; xe2x80x9c2.5 volumesxe2x80x9d means that the volume of carbon dioxide at 0xc2x0 C. under 1 atmosphere is 2.5 times the volume of the liquid. The typical pasteurization cycle, as shown in FIG. 1, includes five steps:
(1) immersion in bath 1, having a bath temperature of about 43xc2x0 C., for about 12.5 minutes in order to raise the container and contents up to about the bath-1 temperature;
(2) immersion in bath 2, having a bath temperature of about 77xc2x0 C., for the time from 12.5 to 21 minutes in order to raise the container and contents up to about the bath-2 temperature;
(3) immersion in bath 3, having a bath temperature of about 73xc2x0 C., for the time from 21 to 31.5 minutes in order to hold the container and contents at about the bath-3 temperature;
(4) immersion in bath 4, having a bath temperature of about 40xc2x0 C., for the time from 31.5 to 43 minutes in order to lower the container and contents down to about the bath-4 temperature; and
(5) immersion in quench bath 5 for the time from 43 to 60 minutes in order to cool the container and contents down to about 10xc2x0 C.
The temperature curve 12 shows that the container and contents remain above 70xc2x0 C. for roughly 10 minutes (in bath 3), during which time the internal pressure curve 14 increases significantly to about 140 psi (1xc3x97106 Nxc2x7mxe2x88x922). This 10-minute hold period at a temperature of about 70 to 75xc2x0 C. provides effective sterilization for most carbonated beverage products, including those containing 100% fruit juice. A glass container can withstand these temperatures and pressures without deformation.
In contrast, a conventional polyester carbonated soft drink (CSD) container made of polyethylene terephthalate (PET), and filled with a carbonated product, would undergo significant volume expansion (creep) when exposed to the elevated temperatures and pressures of the pasteurization process. An exemplary curve 16 of modulus versus temperature for biaxially-oriented PET is shown in FIG. 2. The modulus (an indicator of strength under pressure) decreases with increasing temperature; thus creep increases with increasing temperature. This data shows the tensile properties of a sample taken from a cylindrical panel section of a disposable CSD container made of PET (0.80 IV resin). The panel section was oriented at a planar stretch ratio of about 13:1; the testing was conducted on an Instron machine according to ASTM D638. For this prior art CSD container, the drop in strength at elevated temperatures would result in excessive volume expansion and physical distortion under normal pasteurization conditions, resulting in an unacceptable drop in the fill point and/or base roll out (instability).
PET (homopolymer or copolymer) resin used for disposable CSD containers has a glass transition temperature (Tg) on the order of 65-70xc2x0 C. It is known that increasing the molecular weight (i.e., chain length of PET molecules) of the resin, which effectively increases Tg, can significantly strengthen the resulting biaxially-oriented container so as to resist or diminish creep at elevated temperatures. Intrinsic viscosity (IV) is used in the PET container industry as a standard measure of PET chain length. Known disposable CSD containers (freestanding, monolayer PET containers) have been produced from resins with IVs in the range of 0.70 to 0.85 dl/g. Increasing the IV beyond 0.85, and preferably beyond 0.90, has produced a freestanding monolayer PET container that can be successfully pasteurized at 70-75xc2x0 C. for products carbonated at up to four volumes.
Although a higher molecular weight (higher IV) PET can provide enhanced strength at elevated temperatures, use of such high IV PET is difficult to justify economically because of its cost premium. For example, 0.90 or higher IV PET resins cost 20-30% more per unit weight, than 0.80 IV PET.
FIG. 3 is a graph of modulus versus temperature, similar to FIG. 2, but with three curves 20, 22, 24 to illustrate the influence of IV on the modulus/temperature relationship. Biaxially-oriented PET samples were taken from the panel sections of containers oriented at a planar stretch ratio of 12.0-12.5 for three different resin IVs, namely, 0.74, 0.80, and 1.00. These curves show that for example, at a modulus of 3xc3x97E6 psi (20,690xc3x97106 Nxc2x7mxe2x88x922), there is a temperature difference of 40xc2x0 F. (22.2xc2x0 C.), i.e., 160-120, between the 0.74 IV sample and the 1.00 IV sample. Thus, increasing the IV produces a desirable increase in strength at elevated temperatures, but again at a cost premium.
There is an ongoing need for a plastic container able to withstand the elevated temperatures and pressures of pasteurization and other high temperature applications, and wherein the container can be manufactured commercially at a price competitive with that of glass containers.
The present invention is directed to a multilayer container, which can withstand elevated temperatures and pressures (e.g., the pasteurization process) without significant creep and which is commercially cost-effective. For example, in a preferred embodiment the container undergoes an overall volume increase of no greater than about 3.0%, and more preferably no greater than about 2.0%, compared to the as-molded container volume. The invention is also directed to a method of making the container and to multilayer preforms which are expanded to form containers.
In one embodiment, a two-material, three-layer (2M, 3L) container structure includes exterior inner and outer layers of virgin polyethylene terephthalate (PET ) homopolymer or copolymer, and an interior core layer of post-consumer PET (PC-PET). PC-PET is available at a 15-25% cost advantage, as compared to 0.80 IV virgin PET resin; the cost difference is even greater with virgin PET above 0.80 IV. This savings enables production of a container with 30-60% PC-PET by total container weight, and the remaining 70-40% of 0.85 (or higher) IV virgin PET, that is cost-competitive with existing glass containers for pasteurization. The higher IV outer layers have a higher Tg for enhanced thermal resistance, while the lower IV core provides the necessary wall thickness for strength at a reduced cost.
An unexpected problem arises when preforms are produced with polymers of substantially different IVs, i.e., a difference (delta) of at least 0.10 dl/g, such as a multilayer structure of 0.73 IV PC-PET and 0.85 IV virgin PET. In an IV delta range of 0.10 to 0.20, one or more layers may separate when the container is dropped from a height of one meter onto a hard rigid surface (e.g., concrete). Still further, if the IV delta exceeds 0.20, layer separation may occur in the preform, immediately following removal from the injection mold.
Layer separation is an important commercial issue for CSD containers which are stored for extended periods of time. Carbon dioxide gas may permeate through the container sidewall into a delaminated layer region, creating a pressurized gas pocket; over time, the pocket may expand to a significant size, rendering the container visually unacceptable.
It has been found that the injection molding and/or blow molding process conditions can substantially diminish or completely eliminate the problem of layer separation for IV deltas on the order of 0.10 or more. More specifically, the rate of injection and amount of pressure applied in the preform mold are increased to insure higher levels of layer bonding. For example, a standard injection molding process for low-IV PET (i.e., 0.70 IV) may utilize an injection rate of 10-12 grams/second, and a mold pressure on the order of 7,000 psi (50xc3x97106 Nxc2x7mxe2x88x922). In contrast, the injection rate for molding multilayer virgin PET/PC-PET preforms is increased to about 16-20 grams/second (a 50% or greater increase) and the mold pressure to about 9,000 psi (60xc3x97106 Nxc2x7mxe2x88x922)(about a 30% increase). In a preferred process, the virgin PET is injected at about 16-20 grams/second at a melt temperature of about 275-300xc2x0 C., and the PC-PET is injected at the same rate at a melt temperature of about 265-290xc2x0 C. The mold is then packed (to fill any void space created by shrinkage) at a pressure of about 9000-12,000 psi (60xc3x97106 to 85xc3x97106 Nxc2x7mxe2x88x922), for about 2-3 seconds, and then held (in the mold) at a pressure of about 6000 psi (40xc3x97106 Nxc2x7mxe2x88x922) for about 13-15 seconds before ejection. Still further, the blow molding temperature is preferably about 110xc2x0 C., to minimize inter-layer stresses during blowing.
It is hypothesized that increasing the IV delta between the virgin PET and PC-PET alters the melt solubility of the materials sufficiently to reduce molecular migration and chain entanglement at the layer boundary, thus decreasing layer adhesion. The enhanced injection rate and pressure overcomes this problem. The exact mold temperature, injection rate, pressure and hold time will vary depending upon the specific polymers used and preform wall thicknesses.
The present invention includes multilayer preforms and other injection-molded articles, as well as various containers, including bottles and cans, made from such preforms. The neck finish of the container may be amorphous, biaxially oriented, an insert molded with a high Tg polymer and/or crystallized, depending on the particular wall thickness and/or applications.
These and other advantages of the present invention will be more particularly described in regard to the following detailed description and drawings of select embodiments.